Piezo-electric device and method of producing same



March 19, 1935. c. B. SAWYER 1,995,257

PIEZO ELECTRIC DEVICE AND METHOD OF PRODUCING SAME Filed Feb. 29, 1952 I INVENTOR Charles B. Sawyer B We,

ATTORNEYS Patented Mar. 19, 935

PIEZO-ELECTRIC DEVICE AND METHOD OF PRODUCIN G SAME Charles B. Sawyer, Cleveland Heights, Ohio, as-

signor to The Brush Development Company, Cleveland, Ohio, a corporation of Ohio Application February 29, 1932, Serial No. 595,694

13 Claims.

This invention relates to piezo-electric appa-' 5 trodes are applied to a piezo-electric member in such manner that the acoustical recording or reproduction of the device will be or high quality.

This application is a'continuation in part of my copending application Serial No. 423,284 filed on January 24th, 1930, in which a specific method of applying the electrodes is disclosed and claimed.

One of the objects of this invention is to provide an acoustical device whereby the inherently high specific inductive capacity of Rochelle salt may be more fully utilized.

Another object of this invention is to provide a method of applying an electrode to a piezoelectric memberwhereby the electrode may be associated as closely as possible with the surfac of said member.

A further object of this invention is to provide a method of reducing the impedance between the electrodes of a piano-electric unit substantially to a minimum.

Another object of my invention is to provide an acoustical or other energy translating device comprising a member of Rochelle salt having electrodes attached thereto by an adhesive film, the thickness of the film being insufiicient to affect the quality of the reproduction or recording of the device.

Another object of my invention is to provide an acoustical device comprising a Rochelle salt dielectric and electrodes attached thereto by an adhesive film, the distance between the electrodes and the crystal being such that the capacity between the electrodes will comprise a relatively large proportion of the maximum theoretical capacity.

A further object of my invention is to provide anacoustical reproducing device comprising a Rochelle salt dielectric and electrodes attached thereto by a thin, adhesive film, the thickness of the film being such that the effective separation of an electrode and the crystalline member will not be more than .00005 or an inch.

Another object of my invention is to provide anew method of applying electrodes to Rochelle salt by means of an adhesive film, the thickness of the film being reduced to a minimum so that the impedance between the electrodes shall be low and the high specific inductive capacity of the crystalline material may be more fully utilized.

A still further object of my invention is to provide a new method of applying electrodes to piezo-electric crystals by means. of a thin, adhesive fi lm,.whereby constant electrical properties shall be maintained and a close adhesion will be produced which will not be materially afiected by bending, expansion, contraction, or other dlstortions oi the crystal.

With the foregoing and other objects in view which will be readily apparent from the detailed description to follow and the appended claims, the present invention consists in an improved device that is especially adapted for acoustical apparatus and a method of preparing it which will be readily understood by those skilled in the art to which the invention appertains.

My invention will be better understood by ref-- erence to the accompanying drawing, in which Figure 1 is a portion of a piezo-electric crystal, the electrode shown being partially applied;

Fig. 2 is an exaggerated section of the device shown in Fig. 1; and

Fig. 3 shows an acoustical apparatus in which my improved device may be utilized.

As illustrated in the drawing, the apparatus preferably comprises a slab or portion of Rochelle salt 1 interposed between conducting electrodes 2 which are secured thereto by a thin film of adhesive 3. This in normal operation may act as a piezo-electrical converter of mechanical energy to electrical energy or vice versa, and in general provides a capacative reactance in the circuit to which it is connected.

In Fig. 3 is shown a diagrammatic view of one form of applying a portion of a Rochelle salt crystal to an acoustical device. In this figure the slab of Rochelle salt crystal 1 is mounted on a suitable base 7, and to the opposed faces of the slab are secured the electrodes 2. Upon the portion of the slab opposite the base is secured a clamp 4, which may receive the shaft 5 which carries a suitable loud speaker cone 6.

The method of forming the crystalline slab 1 to operate a cone mounted as indicated in Fig. 3, is fully set forth in my Patent No. 1,802,780, issued April 28, 1931. As clearly set forth in that patent, the slab or plate is clear and has substantially parallel opposite faces and expands and contracts in a direction parallel to said faces. It will be understood that instead of being used as a loud speaker, the crystal may be used in the reverse direction; that is, as a microphone, and it may also be used as a record cutter or a phonograph specification and defined in the appended claims- In applying electrodes to piezo-electric crystals,

' have the tendency to loosen the electrode when it is subjected to alternating electromotive forces and mm the character of the unit, and this is especially true when it is ployed in acoustical devices. In my research upon this subject I have also found that the capacity of the unit varies greatly in accordance with the method or applying the electrode to the crystal and the efiective distance between the electrode and the crystal.

lit has, of course, been heretofore recognized that it is desirable to have the distance between the electrode and dielectric of a condemer small, but practical experience has demonstrated that after the decrease of separation has been reduced to a certain 1c: easily attained for dielectrics such as mica, glass, parafiin paper and the like, a further reduction in separation between the electrode and the dielectric would not result in worthwhile increases in capacity. The di electrics ordinarily utilized in such condensers, however, have a dielectric constant, or specific inductive capacity, ranging between 1 and 12. For example, air has a dielectric constant of l; waxes, paramn, shellac, hard rubber. etc. have a dielectric constant of from 2 to l; and the dielectric constant of glass, mica, etc. will range from d to 10.

When Rochelle salt is used as the dielectric, however, the separation between the crystal and the electrode is of extreme importance. In my investigation upon this subject, in connection with my discovery of the enormously high dielectric constant of Rochelle salt, I have found that the separation that is satisfactory with other dielectrics introduces serious losses in efiiciency when a Rochelle salt crystal, or any substantial equivalent thereof, is utilized as the dielectric. It is, therefore, of vital importance to have the amount of effective separation of the electrode from the dielectric much smaller than necessary or usual in the case of other known dielectrics. When Rochelle salt is used in sound generation or reproduction or in the translation of periodic energy impulses in general, this small separation is of particular importance because instantaneous values of the specific inductive capacity may reach exceedingly high values and are difierent at difierent points of the cycle. For example, even the average dielectric constant of a piezoelectric Rochelle salt plate over the complete cycle has a value greater than 1000 under varying conditions and in most cases is considerably higher, ranging from 2000 to 10,000, and for instantaneous values very much higher. Furthermore, the crystalline part of the device does not act as a pure capacity but has an efiective resistance due to the fact that it is doing work in the translation of energy. Also an appreciable thickness in the separation of the electrode from the crystal has much the same effect as connecting a condenser 1 series with the Rochelle salt element. Consequently separation of the electrode from the crystal element will in effect eccentuate the higher frequency ranges and suppress the lower frequency ranges or otherwise distort the wave form, thereby causing inaccurate reproduction. Since in general the impedance of a condenser is inversely proportional to the capacity of the condenser it is obvious that the impedance due to the film of cement becomes important in relation to the impedance of the crystal portion whenever the eiiective separation between the toil and the crystal portion becomes great enough to make capacity due to the sepmation small in comparison with the capacity of the crystalline portion. Thus the impedance between the elec- 5 trode and the crystalline surface must be small enough so that a considerable variation in its value will not aflect materially the total impedance of the unit. It will, therefore, be understood that the efiective separation of the electrode from the crystalline plate is of special importance when the device is utilized in acoustical apparatus for the generation or the reproduction of sound or in other forms of apparatus for the translation of energy. 15

The deleterious effects of using a thickness of an adhesive between the electrode and the R0- chelle salt dielectric which would be perfectly satisfactory for other kinds of dielectrics, can be best made apparent by an actual example, in which for simplicity the impedances are assumed to be pure capacities.

The capacity of a condenser having the space between the electrodes filled with layers of two dielectrics having difierent thicknesses and difrerent dielectric constants is given by the following formula:

Suppose the dielectric to consist of a piece of glass 0.15" thick and having an area of S sq. in.

and having a dielectric constant or specific inductive capacity of say 6. The electrodes are supposed to be fastened on with a cement having a thickness of 0.0001 of an inch between the electrode and the plate, and a dielectric constant or 2.

Then we have:

equals 39.8074 micromicrofarads. Now if the thickness of the adhesive is reduced to zero, we have:

1 X 6X2 0.2245 (2 X 0.15)+(6X0) 40 micromicrofarads. The increase of 40.039.8074==0.1926 micromicrofarads and 0 equals 0.1926X 39.8074 equals 0.459 percent.

It a Rochelle salt slab be substituted for the 75 glass, however, all other factors remaining the same, we have and the capacity of the condenser is to zero, t: equals and the capacity of the condenser is 1 10000 2 X 0.224s X (2 x 0.is)+ (10000X0) equals I equals 66666.6 micromicrofarads. The increase K in capacity is 66666.668695.6 equals 57970 micromicrofarads, and the percent increase is 5797 0X 100 8695.6 equals 666%.

We thus see that a thickness of an adhesive, or the effective separation of the electrodes from the crystal, resulting in an impedance which will affect the apparent capacity between the electrodes by an unappreciable amount (less than one percent) with ordinary dielectrics may cause a variation of several hundred percent when a material of high dielectric capacity, such as Rochelle salt, is used.-

From the foregoing example, it will also be seen that in order to utilize satisfactorily the inherently high dielectric constant of the Rochelle salt plate in connection with the piezo-electric effect of the plate, the capacity between the contiguous surfaces of the plate and the electrodes must be adequately large compared to the capacity between the parallel surfaces of the plate, or, correspondingly, the impedance between the contiguous surfaces of the plate and the electrodesmust be adequately small. v

Generally speaking, I find in practice that the latter impedance should not exceed that of the Rochelle salt plate and preferably should be substantially smaller in the use of plates as thick as those specifically mentioned below.

The effective separation of the electrode from the crystal may be varied somewhat depending upon the thickness of the crystal. When the crystalline plate is thin it is essential that the separation between the plate and the electrode shall be small, while if the crystalline plate is comparatively thick, somewhat more latitude may be allowed in the degree of separation.

In utilizing plates of Rochelle salt having a thickness of to .2 of an inch, which plates are especially suitable for some forms of apparatus for acoustical reproduction, it has been found that when the effective separation between the electrode and the plate of Rochelle salt crystal is substantially .00005 of an inch or less, highly accurate reproduction is obtained, and, moreover, this separation is one that can be obtained by the herein described methods without excessive labor and production cost. When a unit made in accordance with my invention is employed in acoustical apparatus with the usual acoustical voltages and amount of restraint on the crystal generally applied. good efficiency as well as accurate reproin which D represents the effective distance between the electrodeand the Rochelle crystal plate; K is a constant depending upon the efficiency required; T is the thickness of the crystal; D1 is the dielectric constant of the adhesive film, and D2 is the dielectric constant of the crystal. As previously stated, the degree of separation becomes an increasingly important factor as the dielectric constant of the crystalline mate rial increases, and when a material having a large dielectric constant such as Rochelle salt is employed, a very small separation between the dielectric and the electrode is required. For this reason, D2 appears in the denominator or as a dividing factor in the formula. It has also been ound that the separation need not be so small when the crystalline plate is thicker and also when the dielectric constant of the adhesive is higher; consequently, the thickness of the crystalline plate and the dielectric constant of the adhesive have been made multiplying factors in the formula. A typical value for the constant K is 0.3 when a capacity of at least 60% of the maximum theoretical capacity between the electrodes is required.

Various methods may be utilized for effecting a close adhesion between the electrode and the crystal without unduly increasing the impedance between the electrodes. While it is desirable that the adhesive be thin, a suflicient amount should be applied so that the adhesion between the electrode and the crystal will be substantially unaffected by the mechanical motion of the crystal when it is subjected to alternating electromotive forces.

One very effective method of producing my improved unit is to apply a thin coating of a solution of a gum, such as Canada balsam, in a suitable solvent, such as xylol, benzol or toluol, to the surfaces of the crystal and the electrode. For exampe, 1 gram of dry Canada balsam may be dissolved in about 5 grams of xylol and the adhesive, thus formed, applied either to the surface of the crystalline material or the electrode by means of a brush. Tin foil is a very suitable electrode because it is light and flexible and will not constrain the mechanical movement of the forced into intimate contact by rubbing the electrode until the air bubbles are eliminated and a uniform, thin distribution of the adhesive is produced. Heat is next applied and the electrode is again rubbed when warm to insure the desired smallness of effective separation between the electrode and the crystal surface. It will be understood that if too much adhesive is applied to produce a film of the requisite thinness, the excessive adhesive will be forced from between the crytsalline plate and the electrode during the rubbing or pressing operation.

Another method of applying the electrode is to cover the crystal or tin foil with a thin coating of an adhesive which has the property of contiacting m: derably upon heating. l have found that an adhesive having a cellulosic base, such as nitro-cellulose or cellulose acetate dissolved in a suitable solvent, such as acetone, or a mixture of acetone and alcohol, having the consistency of a light lacquer, is very satisfactory. In app the tin foil to the crystal either the crystal or tin foil is covered with a very thin coat of the adhesive, just sumcient to thoroughly moisten the surface, and the tin foil is forced into intimate contact with the crystal, by rubbing or rolling. lhe cement is then permitted to dry and, because of the contraction of the adhesive, the effective separation between the crystal and the electrode is further reduced.

The term effective separation" is utilized to define themean separation between the crystal and the electrode. Because of the necessary unevenness and departure of the surface of the di electric from a perfect plane, the separation of the electrode and the crystal is different in different places. It may be in actual contact at some points of the surface of the dielectric but separated at other points. This, however, can all be expressed as the equivalent or efiective separation; that is, the uniform separation which would produce the same effect if both surfaces were perfectly plane.

While I have described several specific cements which may be utilized and the method of applying them, it will be understood that I do not desire to be limited to the specific adhesives and methods given as examples, since use may be made of adhesive which is not reactive with Rochelle salt and which when applied will provide sumcient adhesiveness between the electrode and the crystal without increasing the effective separation between them beyond the limit which it is sought to maintain. For example, Japanese size, or other resinous materials dissolved in suitable solvents, may be employed.

From the foregoing description it will be apparent that I have provided an improved unit that has greater capacity and less impedance than previous devices of this type. The effects of my improved construction are especially noticeable in acoustic devices, as it has been found that when a thin film is utilized very superior results are obtained, and this is especially true in the low tonal register. By utilizing one of the improved methods of applying the electrode, a thin adhesive film has been obtained which has the proper adhesion between the crystals and the electrode.

It is to be understood that the carrying out of my invention as herein described, may be varied without departing from the spirit and substance of my invention, the scope of which is commensurate with the appended claims.

What I claim is:

1. In an acoustical device, the combination of a crystalline plate of Rochelle salt and a plurality of electrodes secured thereto by an adhe-' sive cement, the effectiveseparation of the crystal and the electrode being expressed by the following formula hang;

in which D is the effective separation between the electrode and crystalline plate in inches, K is .3 or less, T is the thickness of the crystal in inches, D1 is the dielectric constant of the cement, and D2 is the dielectric constant of the crystal.

2. In an acoustical device, the combination osses? of a cte plate of win t he a substantially unifo thicmess of approtely .l to .2 of an inch and a plurality of electrodes secured to the surfaces thereof, the electrodes and plates having an eflective action of less than .00005 of an inch, whereby the lower frequency ranges new be more accurately reproduced.

3. A piece-electric device comp an. a 1 a line plate of elle salt andan electrode adhesively ated to said plate, the eiilective dis= tance between the electrode and the plate being less M3005 of an inch.

4. A piece-electric device compr 71.. a crystalline plate of Rochelle mlt having a thickness of .1 to .2 of an inch and an electrode adhesively attached to said plate, the efiective distance between the electrode and the plate being less than .00005 of an inch.

5. A piezo-electric device comprising a crystal= line plate of Rochelle mlt and a plurality of electrodes each secured to said plate by a thin of adhesive, the thiclmess' of said being such that the efiective ance between the plate and the electrodes is less than some of an inch.

6. The method of attaching an electrode to a crystalline plate of Rochelle salt which corn= prises applying to a surface of one of the ele= ments a thin coat of adhenve that has the property of contracting upon c, forcing the elec trode and plate together by a slight rubbing action, and then permitting the adhesive to dry.

7. The method of attaching an electrode to a crystalline plate of Rochelle salt which connprises applying to a surface of one of the elements a thin coat of an organic asive that has the property of contracting upon drying, forcing the electrode into contact with the crystal and then permitting the adhesive to dry.

8. The method of attaching an electrode to a crystalline plate of Rochelle salt which comprises applying to a surface of one of the elements a thin coat of an adhesive having a cellulose base, forcing the electrode into contact with the crystal and then permitting the adhesive to dry.

9. The method of attaching an electrode to a crystalline plate of Rochelle salt which comprises applying just sumcient ount of an adhesive having a cellulose base dissolved in a sol= vent to one of the elements to thoroughly moisten the surface thereof, brin the electrode and the crystalline plate into mutual contact and then permitting the solvent to evaporate.

10. A piezo-electric device comprising a crystalline plate of Rochelle salt having substantially parallel opposite faces with the electrical axis substantially perpendicular to said faces, whereby the plate expands and contracts in a, plane substantially parallel to said surfaces and a plurality of electrodes secured to the opposite is of said plate in such a manner that the effective separation of the contiguous surfaces of the plate and the electrodes is equal to, or less than, .3 of

attached to said plate by a thin film of adhesive,

the effective distance between the electrode and the plate being less than .00005 of an inch.

12. A piezo-electric device comprising a 1,995,257 5 crystalline plate of Rochelle salt and an electrode supported adjacent a face of said plate with an supported adjacent a face of said plate with an effective separation between the contiguous sureflfectlve separation between the contiguous surfaces of the plate and the electrode sufiiciently faces of the plate and the electrode of less than small to make the impedance between said sur- .00005 01' an inch. faces less than the impedance of the plate itself. 5

13. A piezo-electric device comprising a crystalline plate of Rochelle salt and an electrode CHARLES B. SAWYER.

CERTIFICATE OF CORRECTION.

March 19, 1935. Patent No. 1,995,257.

CHARLES B. SAWYER.

it is hereby certified that error appears in the printed specification of the above numbered patent requiring correction as follows: Page 4, first column, line 34, after "of" insert the word any; and that the said Letters Patent should be read with this correction therein that the same may conform to the record of the case in the Patent Office.

Signed and sealed this 2nd day of July, A. D. 1935.

Leslie Frazer (Seal) Acting Commissioner of Patents.

CERTIFICATE OF CORRECTION.

March 19, 1935. Patent No. 1,995,257.

CHARLES B. SAWYER.

it is hereby certified that error appears in the printed speeification of the above numbered patent requiring correction as follows: Page 4, first column, line 34, after "of" insert the word any; and that the said Letters Patent should be read with this correction therein that the same may conform to the record of the case in the Patent Office.

Signed and sealed this 2nd day of July, A. D. 1935.

Leslie Frazer (Seal) Acting Commissioner of Patents. 

